After a divisive campaign marked by clashes over race, immigration and other cultural issues, Americans are set to cast votes to determine the balance of power in the US Congress and shape the future of Donald Trump's presidency.

The first national elections since Trump captured the White House in 2016 will be a referendum on the polarising Republican president and his hardline policies, and a test of whether Democrats can turn the energy of the liberal anti-Trump resistance into victories at the ballot box.

"Everything we have achieved is at stake tomorrow," Trump told supporters on Monday night in Fort Wayne, Indiana, at one of his three rallies to stoke turnout on the last day before the election.

All 435 seats in the US House of Representatives, 35 US Senate seats and 36 governorships are up for grabs on Tuesday.

Democrats are favoured by election forecasters to pick up the minimum of 23 House seats they need for a majority, which would enable them to stymie Trump's legislative agenda and investigate his administration.

Republicans are expected to retain their slight majority in the US Senate, currently at two seats, which would let them retain the power to approve US Supreme Court and other judicial nominations on straight party-line votes.

But at least 64 House races remain competitive, according to a Reuters analysis of the three top nonpartisan forecasters, and Senate control was expected to come down to a half dozen close contests in Arizona, Nevada, Missouri, North Dakota, Indiana and Florida.

Democrats also threaten to recapture governor's offices in battleground states such as Michigan, Wisconsin, Ohio and Pennsylvania, a potential help for the party in those states in the 2020 presidential race.

In a last-minute controversy, NBC, Fox News and Facebook on Monday pulled an ad by Trump's campaign that critics had labelled racist.

The 30-second spot featured courtroom video of an illegal immigrant from Mexico convicted in the 2014 killings of two police officers, juxtaposed with scenes of migrants headed through Mexico.

Critics, including members of Trump's own party, had condemned it as racially divisive. CNN already had refused to run the ad, saying it was "racist."

Voter turnout could be the highest for a midterm election in 50 years, experts predicted.

During a six-day blitz to wrap up the campaign, Trump repeatedly raised fears about immigrants, issuing harsh warnings about a caravan of Central American migrants moving through Mexico toward the US border.

A debate about whether Trump's biting rhetoric encouraged extremists erupted in the campaign's final weeks after pipe bombs were mailed to his top political rivals allegedly by a Trump supporter who was arrested and charged, and 11 people were fatally shot at a Pittsburgh synagogue.

But on the eve of the election, the president said in an interview he wished he had a softer tone during his first two years in office - even as he continued his relentless attacks on political rivals.

Trump blamed the political vitriol on election season.

Democratic former President Barack Obama hit the campaign trail in the election's final days to challenge Trump, questioning his policies and character.

"How we conduct ourselves in public life is on the ballot," Obama told Democratic volunteers in suburban Virginia.